13.1 General
This Section should be considered in conjunction with the UK Building Regulations[26]and the relevant Approved Documents, which classify the buildings based on the use of the building and also specify the types of ties that are required in each class. The forces should be considered as accidental loads.
All structures should have a suitable tying system to prevent disproportionate collapse caused by human error or the accidental removal of a member or limited part of the structure or the occurrence of localised damage. This requirement will be satisfied if the following rules are observed.
The structure should have:
■Peripheral ties.
■Internal ties.
■Horizontal ties at columns/walls.
■Vertical ties.
In the design of ties, reinforcement should be provided to carry the tie forces noted in this Section, assuming that the reinforcement acts at its characteristic strength. Reinforcement provided for other purposes may be regarded as providing part or the whole of the required reinforcement.
All ties should be effectively continuous and be anchored at their ends.
13.2 Peripheral ties
At each floor and roof level, an effectively continuous tie should be provided within 1.2 m from the edge. Structures with internal edges (e.g. atria and courtyards) should also have similar peripheral ties.
The peripheral tie should be able to resist a tensile force of:
Ftie,per = (20 + 4n0) kN ≤ 60 kN where n0 = number of storeys
13.3 Internal ties
At each floor and roof level, internal ties should be provided in two directions approximately at right angles.
The internal ties, in whole or in part, may be spread evenly in slabs or may be grouped at or in beams, walls or other positions. If located in walls, the reinforcement should be within 0.5 m of the top or bottom of the floor slabs.
In each direction the tie needs to be able to resist a force, which should be taken as:
Ftie,int= (1/7.5)(gk+ qk)(lr/5)Ft≥ Ft where
(gk+ qk) = average permanent and variable floor actions (kN/m2)
lr = greater of the distances (in m) between centres of the columns, frames or walls supporting any two adjacent floor spans in the direction of the tie under consideration
Ft = (20 + 4n0) ≤ 60 kN (n0is the number of storeys) The maximum spacing of internal ties should be limited to 1.5lr.
9.10.1
9.10.2.2
& NA
9.10.2.3
& NA
13.4 Ties to columns and walls
Columns and walls at the edge and corner of the structure should be tied to each floor and roof.
In corner columns and walls ties should be provided in two directions.
The tie should be able to resist a force of:
Ftie, fac = Ftie, col= maximum (2Ft; lsFt/2.5; 0.03 NEd) where
Ftie,fac = in kN/m run of wall Ftie,col = in kN/column.
Ft = defined in Section 13.3 above ls = floor to ceiling height (in metres)
NEd = total design ultimate vertical load in wall or column at the level considered Tying of external walls is required only if the peripheral tie is not located within the wall.
13.5 Vertical ties
BS EN1992-1-1 requires vertical ties in panel buildings of five storeys or more.
However, relevant current UK Building Regulations require such ties in all buildings that fall into Class 2B and 3 as defined in Section 5 of Approved Document A[27](see Table 13.1). In all such buildings vertical ties should be provided in columns and/or walls.
Each column and wall carrying vertical load should be tied continuously from the lowest to the highest level. The tie should be capable of resisting the load received by the column or wall from any one storey under accidental design situation i.e. using Exp. (6.11b) in BS EN 1990. See Table 2.2e.
Where such ties are not provided either:
■The vertical member should be demonstrated for ‘non-removability’. Non-removability may be assumed if the element and its connections are capable of withstanding a design action at limit state of 34 kN/m2in any direction over the projected area of the member together with the reactions from attached components, which themselves are subject to a loading of 34 kN/m2. These reactions may be limited to maximum reaction that can be transmitted; or
■Each element should be considered to be removed one at a time and an alternative path demonstrated.
Where a column or wall is supported at its lowest level by an element other than a foundation, alternative load paths should be provided in the event of the accidental loss of this element.
9.10.2.4
& NA
9.10.2.5
AD A[27]
PD 6687[7]
9.10.2.5(2)
Table 13.1
Building classes from Approved Document A – structure (2004 edition)[27]
Class Building type and occupancy 1
2A
2B
3
Houses not exceeding 4 storeys Agricultural buildings
Buildings into which people rarely go, provided no part of the building is closer to another building, or area where people do go, than a distance of 1.5 times the building height 5 storey single occupancy houses
Hotels not exceeding 4 storeys
Flats, apartments and other residential buildings not exceeding 4 storeys Offices not exceeding 4 storeys
Industrial buildings not exceeding 3 storeys
Retailing premises not exceeding 3 storeys of less than 2000 m2floor area in each storey Single storey educational buildings
All buildings not exceeding 2 storeys to which members of the public are admitted and which contain floor areas not exceeding 2000 m2at each storey
Hotels, flats, apartments and other residential buildings greater than 4 storeys but not exceeding 15 storeys
Educational buildings greater than 1 storey but not exceeding 15 storeys Retailing premises greater than 3 storeys but not exceeding 15 storeys Hospitals not exceeding 3 storeys
Offices greater than 4 storeys but not exceeding 15 storeys
All buildings to which members of the public are admitted which contain floor areas exceeding 2000 m2but less than 5000 m2at each storey
Car parking not exceeding 6 storeys
All buildings defined above as Class 2A and 2B that exceed the limits on area and/or number of storeys
Grandstands accommodating more than 5000 spectators Buildings containing hazardous substances and/or processes Notes
1 For buildings intended for more than one type of use, the class should be that pertaining to the most onerous type.
2 In determining the number of storeys in a building, basement storeys may be excluded provided such basement storeys fulfil the robustness requirements of Class 2B buildings.